Even in those countries where blood is donated free of charge, as opposed to being purchased, the need to type, screen against various diseases and refrigerate the blood for storage means that there are substantial expenses involved in the making of blood available for use within the health care system. An estimate of the actual cost of supplying a unit (450 ml) of blood is in the vicinity of $A250.00 and approximately 300,000 blood units are used each year in a city such as Sydney, Australia, having a population of 3.5 million. In this connection, the term "blood" is used in relation to blood and blood-like or blood-derived liquids including suspensions of red blood cells (erythrocytes) in saline solution. Such blood-like or blood-derived liquids are in substantial demand for open-heart surgery, car accident victims, difficult births, and like medical procedures.
The ability of blood to be stored depends in large part upon the "life" of the red blood cells within the blood sample. In clinical terms, the poor condition of red blood cells is known as the lack of deformability, the converse of which is known as `brittleness`. The average red blood cell is 7.5 .mu.m in diameter, and in order to pass down capillaries (that are as small as 3 .mu.m) they must readily deform. The inability to deform can reduce the efficiency of circulation. Abnormal deformability has been linked to diseases as varied as sickle cell anaemia and diabetes. It would thus be useful to be able to quickly perform a measurement on a small sample of a person's blood to determine red blood cell deformability and hence the condition of the blood.
Red blood cell deformability increases with the level of fitness in trained athletes, and thus the ability to perform quick measurements of red cell deformability would prove a useful tool to assist in fitness assessment.
There is considerable variability from person to person and from sample to sample so far as blood "shelf-life" is concerned. In some instances, degradation sets in within three to six days, while in other instances there is no appreciable loss of condition in a period up to ten weeks. In the absence of a suitable testing regime, units of blood are normally kept for between three and seven weeks before being discarded at the end of this period if not used.
It is highly desirable that the cost of production of a stock of blood be able to be amortised over a longer period of time, perhaps up to two months, in order to reduce the effective cost of the availability of blood samples. The amortisation period can be increased if the storage period can be increased. The storage period can be increased if a simple and effective test is available which enables a determination to be made as to whether a specific blood sample is of a condition above some suitable minimum condition. If such a test were available, only those blood samples which have actually deteriorated would be quickly discarded, instead of discarding all blood samples on the basis of the "storage life" of those samples which deteriorate fastest.
A particular problem in relation to blood is that once a sample has been sealed within a sterile container, it is highly desirable to maintain the sterility. In order, therefore, to provide a low cost testing method in relation to blood and other such liquids which must be maintained under sterile conditions, it is highly desirable that the test can be applied to the sealed container without breaking the seal in any way. In this way the possibility of contaminating the contents of the sample would be avoided.
It is known from Australian Patent No. 557,256 (which originated from one of the present inventors and is assigned to the present applicant) to measure the flexibility of red blood cells in a liquid suspension by means of passing an ultrasonic signal through the suspension, which is held within a carefully constructed cell. The deformability of red blood cells can be measured using the variation of the absorption of ultrasonic energy with frequency. The walls of the red blood cells have a mechanical relaxation time related to their deformability, and when ultrasound is propagated through blood, some energy is absorbed in deforming the walls due to the relaxation process. There is a near-linear relationship between .alpha./f and f (where .alpha. is the absorption coefficient, and f is frequency).
The method is relatively time consuming in that a number of measurements must be taken at each frequency and also at a number of different frequencies in order to provide the desired results. In addition, laborious impedance matching must be carried out at each individual frequency. The cost of the equipment is approximately $A40-50,000, and the time for a measurement to be made is of the order of two hours, which is too long for use in a commercial blood bank or hospital.
FIG. 14 shows a plot of .alpha./f versus f for five samples of blood in varying condition measured in accordance with the prior art technique discussed above.
In accordance with the prior art method, the blood must be located within the special cell. Therefore it is necessary to destroy the sterile barrier which surrounds a specific blood sample in order to test the sample. In the event therefore that the sample should prove to be satisfactory, it is necessary to re-package the sample and sterilise it again. Naturally this re-sterilisation adds substantially to the practical cost of this prior art method. For the above reasons, the method disclosed in the above mentioned Australian patent has not been used in practice and has not found any commercial success, but instead remains a laboratory research tool.